Recently, as electronic equipment is formed into a compact structure with high reliability, there also has been a growing demand for manufacturing compact and highly reliable aluminum electrolytic capacitors. The trend increases the need for increase in capacitance and decrease in leakage current of anode foil used for aluminum electrolytic capacitors.
Conventionally, some suggestions have been made on the method of manufacturing anode foil used for medium/high-pressure aluminum electrolytic capacitors, such as patent literatures 1 to 4 described below.
For example, the manufacturing method disclosed in patent literature 1 has the following successive steps:                a first step of boiling aluminum foil in pure water;        a second step of dipping the aluminum foil into a solution of weak acid, such as phosphoric acid, acetic acid, citric acid, and succinic acid; and        a third step of performing a chemical conversion treatment on the aluminum foil in a boric-acid solution.        
To decrease leakage current of anode foil, patent literatures 2 and 3 introduce suggestions. In the method of patent literature 2, aluminum foil is boiled in pure water and then dipped into an aqueous solution of an organic acid or a salt thereof—where, the organic acid is formed of straight-chain saturated dicarboxylic acid having an odd number of carbons or trans-straight-chain unsaturated dicarboxylic acid. After that, the aluminum foil undergoes a chemical conversion treatment in an ammonium-adipate aqueous solution. In the method of patent literature 3, first, aluminum foil undergoes a main chemical conversion treatment in a boric-acid solution and then the foil is dipped in a solution of phosphoric acid, boric acid, organic acid, or a salt thereof for a predetermined period of time. After that, the aluminum foil undergoes a follow-up chemical conversion treatment with application of voltage.
Further, in the chemical conversion treatment in patent literature 4, aluminum foil undergoes the treatment with multi-stage application of formation voltage in chemical conversion solutions of citric acid, boric acid, phosphoric acid, and adipic acid. Between the multiple stages of chemical conversion, the anode foil undergoes a depolarization treatment while being dipped into an aqueous solution of citric acid and phosphoric acid.
In the manufacture of anode foil used for medium/high-pressure aluminum electrolytic capacitors, to increase capacitance, it is important that an etched pit on aluminum foil effectively works as an extended surface area to increase capacitance; at the same time, to decrease leakage current, it is important that an oxide film is formed into a structure with no defect as possible.
According to the method of patent literature 1, a hydrated film, which is obtained by the boiling step in pure water, combines with weak acid and has a stable structure, suppressing leakage current in a stable condition. However, the weak acid can damage the hydrated film, which increases defects in a film to be formed in the next process. As a result, noticeable decrease in leakage current cannot be achieved.
To address the problem above, according to the method disclosed in patent literature 2, prior to the chemical conversion treatment, aluminum foil is dipped into an aqueous solution of straight-chain saturated dicarboxylic acid or trans-straight-chain unsaturated dicarboxylic acid. This allows straight-chain saturated dicarboxylic acid to be attached to the outer layer of the hydrated film obtained by the boiling process in pure water, suppressing dissolution of the hydrated film into the solution, and accordingly, suppressing leakage current.
However, in general understanding, an electric field exerted on the chemical film accelerates crystallization of the film. It has believed that the crystallization invites contraction in volume, resulting in a defective film. Therefore, sufficient reduction in defective films is not expected, that is, high reliability is not achieved, although leakage current is suppressed smaller than the method of patent literature 1.
According to the method of patent literature 3, aluminum foil undergoes a thermal depolarization treatment after the main chemical conversion, and then the foil is dipped into a solution for the follow-up chemical conversion for a predetermined period of time. After that, the foil undergoes the follow-up chemical conversion with application of voltage. Through the processes above, a defective portion of the film is exposed and repaired by the follow-up chemical conversion treatment, which contributes to decrease in leakage current. After a hydration treatment, aluminum foil undergoes chemical conversion, until voltage reaches a predetermined level, in a boric-acid solution where aluminum is hard to dissolve. Therefore, of the etched pits on the foil, a narrow pit is inconveniently embedded in the chemical film. As a result, an effective surface area to the actually etched pits is not sufficient, which has been an obstacle to attaining high capacitance.
Besides, in the method above, a depolarization treatment or an acid treatment is employed for repairing defects in the film due to crystallization of oxide films. In the depolarization treatment with heat, a thermally oxidized film formed during the thermal process covers openings of the defective film. This hampers penetration of a chemical solution into the film in the follow-up chemical conversion. Such an insufficient chemical conversion cannot decrease leakage current and results in poor reliability. In the acid treatment, the openings of the defective film are properly kept in size; at the same time, a chemical film formed in a boric-acid solution only is relatively easy to dessolve in acid, which invites an excessive dissolution of the oxidized film. In this case, too, leakage current is not sufficiently decreased, which results in poor reliability.
According to the method of patent literature 4, aluminum foil undergoes a chemical conversion treatment formed of multi-stage processes at different formation voltages, while having a depolarization treatment between the multi-stage processes. The method brings improvements in decreasing leakage current. However, dissolution, dehydration, or transformation occurs in the hydrated film because the depolarization treatment is carried out before voltage reaches a value of final formation stage. As a result, the chemical film is formed into a low-crystalline oxide film, by which high capacitance is not expected.
Patent Literature
    Patent literature 1; Japanese Unexamined Patent Application Publication No. 59-89796    Patent literature 2: Japanese Unexamined Patent Application Publication No. H06-275473    Patent literature 3: Japanese Unexamined Patent Application Publication No. H10-223483    Patent literature 4: Japanese Unexamined Patent Application Publication No. H02-128415